Charged particle beam microscopy, such as scanning ion microscopy and electron microscopy, provides significantly higher resolution and greater depth of focus than optical microscopy. In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary electron beam. The secondary electrons are detected, and an image is formed, with the brightness at each point on the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. Scanning ion microscopy (SIM) is similar to scanning electron microscopy, but an ion beam is used to scan the surface and eject the secondary electrons.
In a transmission electron microscope (TEM), a broad electron beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time consuming work. The term “TEM” sample as used herein refers to a sample for either a TEM or an STEM and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM.
TEM samples are typically less than 100 nm thick, but for some applications samples must be considerably thinner Thickness variations in the sample result in sample bending, over-milling, or other catastrophic defects. For such small samples, preparation is a critical step in TEM analysis that significantly determines the quality of structural characterization and analysis of the smallest and most critical structures.
FIG. 1 shows one type of typical TEM sample holder 100 commonly called a “grid”, which comprises a partly circular 3 mm ring. In some applications, a sample 104 is attached to a finger 106 of the TEM grid by ion beam deposition or an adhesive. The sample extends from the finger 106 so that in a TEM (not shown) an electron beam will have a free path through the sample 104 to a detector under the sample. The TEM grid is typically mounted horizontally onto a sample holder in the TEM with the plane of the TEM grid (and thus the plane of the attached sample) perpendicular to the electron beam, and the sample is observed.
FIG. 2 shows a cross-sectional view of TEM sample 200 that is partly extracted from a substrate or work piece 202 using a typical process. An ion beam 204 cuts trenches 206 and 208 on both side of sample to be extracted, leaving a thin lamella 210 having a major surface 212 that will be observed by an electron beam. The sample 200 is then freed by tilting the work piece 202 in relation to an ion beam, and cutting around its sides and bottom. A probe 216 attaches to the top of the sample 200, before or after it is freed, and transports the sample to a TEM grid. FIG. 2 shows sample 200 almost entirely freed, remaining attached by a tab 218 on one side. FIG. 2 shows ion beam 204 ready to sever tab 218.
TEM samples can be broadly classified as “cross-sectional view” samples or “planar view” samples, depending on how the sample was oriented on the work piece. If the face of the sample to be observed was parallel to the surface of the work piece, the sample is referred to as a “planar view” or “plan view” sample. If the face to be observed was perpendicular to the work piece surface, the sample is referred to as a “cross-sectional view” sample.
FIG. 3 shows a substrate or work piece 300 from which a cross-sectional view sample 302 is being extracted. The sample 302 is undercut by two intersecting ion beam cuts 306A and 306B from opposite directions, and then the ion beam cuts the sides 308A and 308B to free a “chunk” or large sample that requires additional thinning before observation. A probe 310 is attached to the top surface of the sample 304. The extracted sample is therefore oriented horizontally. With the sample attached in a horizontal orientation to a vertically oriented TEM grid, the sample extends normal to the plane of the grid, and the top surface of the sample 304 is unobstructed for thinning from the top side with a FIB.
Thinning a TEM sample from the top side is commonly called “top down” thinning A significant problem for the preparation of TEM samples from the top side is commonly referred to as “curtaining.” Curtaining is most often observed in semiconductor materials where multiple patterned layers of materials having a low sputtering yield blocks a faster sputtering yield material. Curtaining may also be observed in materials exhibiting different topographic regions where changes in sputtering yields vary with the milling incident angle. FIB thinning of a sample having these types of structural or density variations will cause a “curtain” to propagate from the bottom of the density-variation structure (i.e. metal line) down the face of the milled cross-section. Curtaining artifacts reduce the quality of the TEM imaging and limit the minimal useful specimen thickness. For ultra-thin TEM samples, defined herein as samples having a thickness of less than 30 nm, the two cross-section faces are obviously in very close proximity so thickness variations from curtaining effects can cause a sample to be unusable.
In order to minimize curtaining in TEM sample preparation, it is known to invert the sample so that the bottom or backside of the sample (bulk silicon) is facing the FIB column. Because the bulk portion of the sample will not have imbedded features such as metal lines or transistors, curtaining artifacts will not be introduced into the portion of the sample face containing the region of interest, i.e., the layers of circuitry on the top surface of the semiconductor. While this technique works reasonably well in the preparation of TEM samples, it is difficult to expose and thin the backside of a cross-sectional sample in a conventional FIB system. In systems without an expensive flipstage, often two or even three separate probe manipulations and welds are required to invert the sample without venting and unloading the vacuum. Prior art techniques and devices for accomplishing the sample inversion either require expensive additional equipment, or time-consuming additional manipulation and welding steps, or even manual sample manipulation outside vacuum.
What is needed is an improved method for TEM sample preparation including backside thinning that can be used with conventional sample stages without the use of expensive additional equipment and that can be performed more rapidly and without breaking vacuum.